This application is a continuation of international Application No. PCT/US03/22550, filed on Jul. 18, 2003, which claims the benefit of U.S. Provisional Application No. 60/397,380, filed Jul. 19, 2002, now abandoned.

The present invention relates to novel chimeric nicotinic acetylcholine receptor (nAChR) subunits. Isolated nucleic acid molecules are provided encoding chimeric nAChR subunits based on substituted human nAChR subunits, including chimeric human α4, β2, and β4 nAChR subunits. Chimeric nAChR subunit polypeptides are also provided, as are vectors, host cells and recombinant methods for producing the same. The invention further relates to screening methods for identifying modulators of nAChR activity using the chimeric subunits of the invention.

Each nAChR subtype is a homo- or hetero-pentameric assembly of distinct subunits. Subunit N-terminal domains contribute to ligand recognition, and second transmembrane domains lining the ion channel contribute to channel kinetics and ion selectivity. Large, second cytoplasmic domains (C2) contain sequences that are unique to each subunit including possible phosphorylation sites implicated in nAChR desensitization.

Exploration of nAChR subunit function in receptor activity is important in order to further understand modulation and possible control of receptor function in vivo. Nicotine has been proposed to have a number of pharmacological effects. See, for example, Pullan et al., N. Engl. J. Med. 330:811 (1994). Certain of those effects can be related to effects upon neurotransmitter release. See, for example, Sjak-shie et al., Brain Res. 624:295 (1993), where neuroprotective effects of nicotine are proposed. Release of acetylcholine and dopamine by neurons upon administration of nicotine has been reported by Rowell et al., J. Neurochem. 43:1593 (1984); Rapier et al., J. Neurochem. 50:1123 (1988); Sandor et al., Brain Res. 567:313 (1991) and Vizi, Br. J. Pharmacol. 47:765 (1973). Release of norepinephrine by neurons upon administration of nicotine has been reported by Hall et al., Biochem. Pharmacol. 21:1829 (1972). Release of serotonin by neurons upon administration of nicotine has been reported by Hery et al., Arch. Int. Pharmacodyn. Ther. 296:91 (1977). Release of glutamate by neurons upon administration of nicotine has been reported by Toth et al., Neurochem Res. 17:265 (1992).

Confirmatory reports and additional recent studies have included the modulation in the Central Nervous System (CNS) of glutamate, nitric oxide, GABA, takykinins, cytokines and peptides (reviewed in Brioni et al., Adv. Pharmacol. 37:153 (1997)). In addition, nicotine reportedly potentiates the pharmacological behavior of certain pharmaceutical compositions used for the treatment of certain disorders. See, for example, Sanberg et al., Pharmacol. Biochem. & Behavior 46:303 (1993), Harsing et al., J. Neurochem. 59:48 (1993) and Hughes, Proceedings from Intl. Symp. Nic. S40 (1994). Furthermore, various other beneficial pharmacological effects of nicotine have been proposed. See, for example, Decina et al., Biol. Psychiatry 28:502 (1990); Wagner et al., Pharmacopsychiatry 21:301 (1988); Pomerleau et al., Addictive Behaviors 9:265 (1984); Onaivi et al., Life Sci. 54(3):193 (1994); Tripathi et al., J. Pharmacol. Exp. Ther. 221:91(1982); and Hamon, Trends in Pharmacol. Res. 15:36 (1994).

Various nicotinic compounds have been reported as being useful for treating a wide variety of conditions and disorders. See, for example, Williams et al., Drug News Perspec. 7(4):205 (1994); Arneric et al., CNS Drug Rev. 1(1):1 (1995); Arneric et al., Exp. Opin. Invest. Drugs 5(1):79 (1996); Bencherif et al., J. Pharmacol. Exp. Ther. 279:1413 (1996); Lippiello et al., J. Pharmacol. Exp. Ther. 279:1422 (1996); Damaj et al., J. Pharmacol. Exp. Ther. 291:390 (1999); Chiari et al., Anesthesiology 91:1447 (1999); Lavand'homme and Eisenbach, Anesthesiology 91:1455 (1999); Holladay et al., J. Med. Chem. 40(28):4169 (1997); Bannon et al., Science 279:77 (1998); PCT WO 94/08992, PCT WO 96/31475, PCT WO 96/40682, and U.S. Pat. No. 5,583,140 to Bencherif et al., U.S. Pat. No. 5,597,919 to Dull et al., U.S. Pat. No. 5,604,231 to Smith et al. and U.S. Pat. No. 5,852,041 to Cosford et al. Nicotinic compounds are reported as being particularly useful for treating a wide variety of CNS disorders. Indeed, a wide variety of compounds have been reported to have therapeutic properties. See, for example, U.S. Pat. No. 5,187,166 to Kikuchi et al.; U.S. Pat. No. 5,672,601 to Cignarella; PCT WO 99/21834; PCT WO 97/40049; UK Patent Application GB 2295387; and European Patent Application 297,858.

CNS disorders are a type of neurological disorder. Several CNS disorders can be attributed to a deficiency of acetylcholine, dopamine, norepinephrine and/or serotonin. CNS disorders can be drug-induced; can be attributed to genetic predisposition, infection or trauma; or can be of unknown etiology. CNS disorders comprise neuropsychiatric disorders, neurological diseases and mental illnesses, and include neurodegenerative diseases, behavioral disorders, cognitive disorders and cognitive affective disorders. There are several CNS disorders whose clinical manifestations have been attributed to CNS dysfunction (i.e., disorders resulting from inappropriate levels of neurotransmitter release, inappropriate properties of neurotransmitter receptors, and/or inappropriate interaction between neurotransmitters and neurotransmitter receptors). Relatively common CNS disorders include pre-senile dementia (early-onset Alzheimer's disease), senile dementia (dementia of the Alzheimer's type), micro-infarct dementia, AIDS-related dementia, Creutzfeld-Jakob disease, Pick's disease, Parkinsonism including Parkinson's disease, progressive supranuclear palsy, Huntington's chorea, tardive dyskinesia, hyperkinesia, mania, attention deficit disorder, anxiety, dyslexia, schizophrenia, depression, obsessive-compulsive disorders and Tourette's syndrome.

It is desirable to provide methods for the prevention and treatment of a condition or disorder by administering a nicotinic compound to a patient susceptible to or suffering from such a condition or disorder. It would be highly beneficial to provide individuals suffering from certain disorders (e.g., CNS diseases) with interruption of the symptoms of those disorders by the administration of a pharmaceutical composition containing an active ingredient having nicotinic pharmacology and which has a beneficial effect (e.g., upon the functioning of the CNS), but which does not provide any significant associated side effects. It would be highly desirable to provide a pharmaceutical composition incorporating a compound which interacts with nicotinic receptors, such as those which have the potential to effect the functioning of the CNS, but, when employed in an amount sufficient to effect the functioning of the CNS, does not significantly effect those receptor subtypes which have the potential to induce undesirable side effects (e.g., appreciable activity at cardiovascular and skeletal muscle sites). The discovery and development of such compositions and methods depends on knowledge of receptor activity, including ligand-modulation of receptor activity through ligand interaction with the receptor or specific subunits thereof.

Accordingly, there is a need for further elucidation of nAChR activity and the modulation thereof. Modified receptor subunits provide additional means to explore modulation of receptor activity. Therefore, there is a need for identification and characterization of modified receptor subunits.

The present invention provides isolated nucleic acid molecules comprising a polynucleotide encoding the chimeric nAChR subunit polypeptides having the amino acid sequence shown in FIG. 3, 5, or 7 (SEQ ID NOS: 7, 9, or 11, respectively), or any other sequence according to the present invention.

The present invention also relates to recombinant vectors, which include the isolated nucleic acid molecules of the present invention, and to host cells containing the recombinant vectors, as well as to methods of making such vectors and host cells and for using them for production of chimeric nAChR subunit polypeptides or peptides by recombinant techniques.

The invention further provides an isolated chimeric nAChR subunit polypeptide having an amino acid sequence encoded by a polynucleotide described herein.

The present invention also provides a screening method for identifying compounds capable of enhancing or inhibiting a cellular response mediated by a chimeric nAChR subunit, which involves contacting cells which express a chimeric nAChR subunit with the candidate compound, assaying a cellular response, and comparing the cellular response to a standard cellular response, the standard response being assayed when contact is made in absence of the candidate compound; whereby, an increased cellular response over the standard indicates that the compound is an agonist and a decreased cellular response over the standard indicates that the compound is an antagonist, i.e. for the function measured by the assay.

As used herein the term “chimeric nAChR subunit” polypeptide includes human nAChR subunits having a substitution of an non-naturally occurring sequence in the region of the polypeptide including at least a part of a second, large cytoplasmic domain (C2).

Accordingly, in one aspect, the invention relates to a polypeptide comprising a chimeric human nAChR receptor subunit having a substitution of at least a portion of the C-terminal cytoplasmic domain, wherein the substitution comprises at least about 15% of the native amino acid sequence of the subunit. The substitution can also comprise at least about 20% of the native amino acid sequence of the subunit. The substitution can also comprise at least a portion of a transmembrane domain adjacent to the C-terminal end of the cytoplasmic domain.

In one embodiment, the substitution further comprises all of the transmembrane domain C-terminal to the large, C-terminal cytoplasmic domain and at least a portion of a C-terminal extracellular domain. In other embodiments, the substitution further comprises at least a portion of a transmembrane domain adjacent to the N-terminal end of the cytoplasmic domain. In still further embodiments, the substitution comprises all of the C-terminal cytoplasmic domain and at least a portion of each transmembrane domain positioned adjacent to the cytoplasmic domain. In another embodiment, the substitution further comprises all of the transmembrane domains adjacent to the cytoplasmic domain and all native subunit amino acids C-terminal to the cytoplasmic domain.

The substituted sequence can be characteristic of domains that are structurally analogous to those being replaced in the native nAChR polypeptide and can be derived from a receptor subunit of a ligand-gated ion channel superfamily receptor. The receptor subunit from which the substituted sequence is derived can be characterized by having four transmembrane domains. The subunit can be from a receptor selected from the group consisting of GABA-A, glycine, seratonin, and nAChR receptors. The receptor can be a homopentameric receptor. The substituted sequence can be derived from a nAChR receptor subunit. The nAChR receptor subunit can be an α7 nAChR subunit. The substituted sequence can also be derived from a subunit of a serotonin type 3 receptor. The substituted sequence can also be derived from a subunit selected from the group consisting of GABA-A R α3, GABA-A R β1, glyR α3, and glyR β.

In another aspect, the invention relates to a polynucleotide encoding the polypeptide disclosed herein or a polypeptide having conservative amino acid substitutions thereof. In one embodiment, the polynucleotide is DNA.

In another aspect, the invention relates to a method of making a vector comprising inserting the polynucleotide of the invention into a vector.

In another aspect, the invention relates to a vector produced by the method of the invention.

In another aspect, the invention relates to a method of making a host cell comprising introducing the vector of the invention into a host cell.

In another aspect, the invention relates to a host cell produced by the method of the invention.

In another aspect, the invention relates to an isolated polypeptide of the invention, produced by a method comprising: (a) introducing a vector comprising a polynucleotide encoding the polypeptide into a host cell; (b) culturing the host cell; and (c) recovering the polypeptide.

In another aspect, the invention relates to a method for producing a polypeptide comprising: (a) culturing the host cell of the invention under conditions that the vector is expressed; and (b) recovering the polypeptide.

In another aspect, the invention relates to cells containing at least one polynucleotide of the invention, wherein said cells are bacterial cells, eukaryotic cells or amphibian oocytes. In one embodiment of the invention, the cells further contain at least one polynucleotide encoding a second subunit of human nAChR, wherein the subunit can be a chimeric or native amino acid sequence and is a α subunit if the first subunit is a β subunit and is a β subunit if the first subunit is an α subunit. The cells can be further characterized as being capable of expressing voltage dependent calcium channels. Also, the cells can be characterized as expressing functional nAChR that contain one or more subunits encoded by the polynucleotide.

In another aspect, the invention relates to a method of screening compounds to identify compounds which modulate the activity of human neuronal nAChR. The method comprises determining the effect of a compound on the neuronal nAChR activity in test cells, compared to the effect on control cells or to the neuronal nAChR activity of the cells in the absence of the compound, wherein control cells are substantially identical to the test cells, but control cells do not express nAChR.

In another aspect, the invention relates to a method of screening compounds to identify compounds which modulate the activity of human neuronal nAChR, said method comprising determining the effect of a compound on the neuronal nAChR activity in test cells compared to the effect on control cells or to the neuronal nAChR activity of the cells in the absence of the compound, wherein control cells are substantially identical to the test cells, but control cells do not express nAChR.

It will be recognized that compounds screened according to methods of the present invention can be characterized as agonists, antagonists, or partial agonists based on evaluation of their interaction with nAChR comprising the chimeric subunits of the invention. Also, compounds can be screened on the basis of their binding or more limited functional effects such that candidates for further evaluation are determined. In this aspect, the methods of the invention can provide an initial screen for compounds that may be further evaluated based on other experiments. The invention provides methods for characterization of compounds as agonists, antagonists, or partial agonists, as well as methods for initial selection of candidate compounds for further evaluation.

In another aspect, the invention relates to a polynucleotide encoding a human nAChR α4 subunit comprising a substitution of at least a portion of the large C-terminal cytoplasmic domain, the substitution comprising at least about 15% of the native amino acid sequence of the subunit. The substitution can comprise at least about 20% of the native sequence. In one embodiment, the substitution of the subunit begins in a region encoding from about amino acid position number P304 to about amino acid position number S362 of SEQ ID NO: 1 and ends in a region beginning from about amino acid number P562 to about I627 of SEQ ID NO:1. In another embodiment, the substitution of the subunit begins in a region encoding from about amino acid position number H331 to about amino acid position number L355 of SEQ ID NO: 1 and ends in a region beginning from about amino acid number R566 to about R600 of SEQ ID NO:1.

In another aspect, the invention relates to a polynucleotide encoding a human nAChR β2 subunit comprising a substitution of at least a portion of the large C-terminal cytoplasmic domain, the substitution comprising at least about 15% of the native amino acid sequence of the subunit. The substitution can comprise at least about 20% of the native sequence. In one embodiment, the substitution of the subunit begins in a region encoding from about amino acid position number P295 to about amino acid position number R353 of SEQ ID NO: 2 and ends in a region beginning from about amino acid number C422 to about K502 of SEQ ID NO:2. In another embodiment, the substitution of the subunit begins in a region encoding from about amino acid position number H322 to about amino acid position number Q350 of SEQ ID NO: 2 and ends in a region beginning from about amino acid number E426 to about R460 of SEQ ID NO:2.

In another aspect, the invention relates to a polynucleotide encoding a human nAChR β4 subunit comprising a substitution of at least about 15% of the native amino acid sequence of the subunit large C-terminal cytoplasmic domain. The substitution can comprise at least about 20% of the native sequence. In one embodiment, the substitution of the subunit begins in a region encoding from about amino acid position number P293 to about amino acid position number G351 of SEQ ID NO: 3 and ends in a region beginning from about amino acid number Q422 to about D498 of SEQ ID NO:3. In another embodiment, the substitution of the subunit begins in a region encoding from about amino acid position number H320 to about amino acid position number K348 of SEQ ID NO: 3 and ends in a region beginning from about amino acid number E426 to about R460 of SEQ ID NO:3.

In will be recognized that the amino acid positions above, noted with respect to the particular sequences disclosed herein, will have appropriate counterpart positions in homologous regions of related subunits. Accordingly, one of skill in the art will be able to use this guidance in selecting appropriate positioning for other substitutions according to the invention disclosed herein.

Nicotinic AChR are diverse members of the ligand-gated ion channel superfamily (Lukas, R. J., Neuronal nicotinic acetylcholine receptors, in The Nicotinic Acetylcholine Receptors: Current Views and Future Trends, F. J. Barrantes, Editor., Springer Verlag, Berlin/Heidelberg and Landes Publishing: Georgetown, Tex. p. 145-173 (1998)). Each nAChR subtype is a pentamer assembled as a unique combination of diverse subunits encoded by a member of a family of at least 17 genes. Each of the nAChR subunits has a conserved structure including an extracellular N-terminal domain, four transmembrane domains (M1-M4), a large cytoplasmic loop located between the M3 and M4 domains, and an extracellular C-terminus. Sequences and structural elements involved in ligand recognition are embedded in the extracellular N-terminal domain (Devillers-Thiery, A., et al., J Membr Biol., 136(2):97-112 (1993)) and the small extracellular loop between M2 and M3 transmembrane domains (Campos-Caro, A., et al., Proc Natl Acad Sci USA, 93(12):6118-23 (1996)). Furthermore, the M2 transmembrane domain is thought to form the lining of the ion-channel.

Without wishing to be bound by any particular theory, it appears that the structural characteristics of receptors comprising the chimeric subunits relate to their functional characteristics in a manner that is partially predictable based on preserved or substituted domains of the particular chimera. The chimeras of the invention preserve the first extracellular domain involved in subunit assembly and ligand recognition. These chimeras preserve the drug-binding properties of the native receptors. Also, the first and second transmembrane domains, and even the short loop between M2 and M3, preserve gating characteristics of the channel (lined by M2 and—in part, at least in the “vestibule” region on the extracellular face by M1 sequences) and transduction of ligand binding to channel opening (believed to involve interactions between the external domain, the short M2-M3 loop, and the external surfaces of the channel). In the chimeric subunits of the invention, the artificially introduced cytoplasmic +/−M3/M4 sequences can confer unique properties to the engineered subunits of the source subunit. Among these might be selective targeting of the subunit to specific cellular domains, specific interactions with cytoplasmic or cytoskeletal proteins that could be used to help purify the chimeric receptors or subunits, and any unique functional properties.

In FIGS. 8A-8C, protein sequences from the third transmembrane domain through to the C-terminus are aligned for human nicotinic acetylcholine receptor (nAChR) α4, β2, and β4 subunits, human and mouse (mur) serotonergic 5-HT3 subunits (5-HT3), human γ-amino butyric acid A receptor (GABA-A R) α3 and β1 subunits, and human glycine receptor (glyR) α3 and β subunits. Amino acid residue numbering makes reference to the translation initiation methionine as residue 1. Sequences from these proteins are representative of those that can be used to generate chimeric subunits, assembly of which can produce novel receptors having ligand binding and ion channel properties dictated by N-terminal extracellular and first and second transmembrane domains but that can have novel features dictated by third and fourth transmembrane domains and the second, major, cytoplasmic loop.

Potential starting positions for splicing sequences from one subunit into another are indicated by the dark shading, and potential stopping positions for such insertions are indicated by light shading in FIGS. 8A-8C. For example, substitution for the nAChR α4 sequence starting at the beginning of the M3 domain (P304) could be effected using sequences starting at the corresponding amino acid residue of other subunits (e.g., P295 from the nAChR β subunit, P303 from the human 5-HT3 subunit, or A315 from the glyR α3 subunit). Substitution for the nAChR α4 sequence starting anywhere in the C2 domain bordering M3 (e.g., from I323 to S362) could be effected using sequences starting at the corresponding amino acid residues of other subunits (e.g., from T313 to S352 for the nAChR β2 subunit, from E322 to R358 for the human 5-HT3 subunit, or from E333 to R352 for the glyR α3 subunit). As other examples, aside from continuing the substitution all the way through to the C-terminal residue, substitution for the nAChR α4 sequence ending anywhere in the C2 domain bordering M4 (e.g., from P572 to R600) could be effected using sequences starting at the corresponding amino acid residues of other subunits (e.g., from G423 to R460 for the nAChR β2 subunit, from G417 to R451 for the human 5-HT3 subunit, or from P414 to R433 for the glyR α3 subunit).

In one example of the present invention, the substituted sequence is derived from a serotonin type 3 receptor (5HT3R). The 5HT3R receptors are also members of the ligand-gated ion channel superfamily, and share extensive sequence similarity and, also likely, structural homology with nAChR (Maricq, A. V., et al., Science, 254(5030):432-7 (1991)). In fact, the nAChR-5HT3 chimera of the invention, containing the nAChR subunit sequences from N-terminal extracellular domain to the M2 domain, generate chimera receptors possessing at least some of the pharmacological characteristics of the given nAChR subunit. Further, chimeras containing only the 5HT3 sequence region within the large cytoplasmic loop located between the M3 and M4 preserve all the conserved transmembrane domains of nAChR subunits and appear to preserve most pharmacological characteristics of the native subunits.

The chimeras of the invention allow basic evaluation of the functional roles of the cytoplasmic loop. It has been noted that the chimeras exhibit differences in “acute desensitization,” which makes reference to the rate with which the magnitude of the inward current flowing into the cell decreases during agonist application. Use of such chimeras in particular assay protocols presents certain advantages, e.g. for comparative purposes.

In certain embodiments, eukaryotic cells with nAChRs having chimeric subunits according to the invention are produced by introducing into the cell a first composition, which contains at least one RNA transcript that is translated in the cell into a subunit according to the invention. The subunits that can be translated include an α subunit of a human neuronal nicotinic AChR. The composition that is introduced can contain an RNA transcript which encodes an α subunit and also contains an RNA transcript which encodes a β subunit of a human neuronal nicotinic AChR. RNA transcripts can be obtained from cells transfected with DNAs encoding receptor subunits or by in vitro transcription of subunit-encoding DNAs. Methods for in vitro transcription of cloned DNA and injection of the resulting mRNA into eukaryotic cells are well known in the art. Amphibian oocytes are particularly preferred for expression of in vitro transcripts of the nAChR subunits provided herein. See, for example, Dascal, CRC Crit. Rev. Biochem. 22:317-387 (1989), for a review of the use of Xenopus oocytes to study ion channels.

Thus, pairwise (or stepwise) introduction of DNA or RNA encoding α and β subtypes into cells is possible. The resulting cells can be tested by the methods provided herein or known to those of skill in the art to detect functional AChR activity. Such testing will allow the identification of pairs of α and β subunit subtypes that produce functional AChRs, as well as individual subunits that produce functional AChRs.

As used herein, activity of a nicotinic AChR refers to any activity characteristic of an nAChR. Such activity can typically be measured by one or more in vitro methods, and frequently corresponds to an in vivo activity of a nicotinic AChR. Such activity can be measured by any method known to those of skill in the art, such as, for example, measuring the amount of current which flows through the recombinant channel in response to a stimulus.

Methods to determine the presence and/or activity of nicotinic AChRs include assays that measure nicotine binding, ⁸⁶Rb ion-flux, Ca²⁺ influx, the electrophysiological response of cells, the electrophysiological response of oocytes transfected with RNA from the cells, and the like. In particular, methods are provided herein for the measurement or detection of an AChR-mediated response upon contact of cells containing the DNA or mRNA with a test compound.

As used herein, a functional nicotinic AChR is a receptor that exhibits an activity of nicotinic AChRs as assessed by any in vitro or in vivo assay disclosed herein or known to those of skill in the art. Possession of any such activity that can be assessed by any method known to those of skill in the art and provided herein is sufficient to designate a receptor as functional. Because all combinations of α and β subunits may not form functional receptors, numerous combinations of α and β subunits can be tested in order to fully characterize a particular subunit and cells which produce same. Thus, as used herein, “functional” with respect to a recombinant or heterologous nicotinic AChR means that the receptor channel is able to provide for and regulate entry of nicotinic AChR-permeable ions, such as, for example, Na⁺, K⁺, Ca²⁺ or Ba²⁺, in response to a stimulus and/or bind ligands with affinity for the receptor. Preferably such nicotinic AChR activity is distinguishable, such as by electrophysiological, pharmacological and other means known to those of skill in the art, from any endogenous nicotinic AChR activity that may be produced by the host cell.

In accordance with a particular embodiment of the present invention, chimeric nicotinic AChR subunit-expressing mammalian cells or oocytes can be contacted with a test compound, and the modulating effect(s) thereof can then be evaluated by comparing the AChR-mediated response in the presence and absence of test compound, or by comparing the AChR-mediated response of test cells, or control cells (i.e., cells that do not express nAChRs), to the presence of the compound. Of course, effects of test compounds can be evaluated competitively in comparison to compounds known to modulate the receptors comprising the relevant subunits. Test compounds can also be evaluate based on their differential effect on receptors that comprise differing chimeric nAChR subunits.

As used herein, a compound or signal that “modulates the activity of a nicotinic AChR” refers to a compound or signal that alters the activity of nAChR so that activity of the nAChR is different in the presence of the compound or signal than in the absence of the compound or signal. In particular, such compounds or signals include agonists, antagonists, and partial agonists. The term agonist refers to a substance or signal, such as ACh, that activates receptor function; and the term antagonist refers to a substance that interferes with receptor function. Typically, the effect of an antagonist is observed as a blocking of activation by an agonist. Antagonists include competitive and non-competitive antagonists. A competitive antagonist (or competitive blocker) interacts with or near the site specific for the agonist (e.g., ligand or neurotransmitter) for the same or closely situated site. A non-competitive antagonist or blocker inactivates the functioning of the receptor by interacting with a site other than the site that interacts with the agonist.

An “agonist” can be a substance that activates its binding partner. Activation can be defined in the context of the particular assay, or may be apparent in the literature from a discussion herein that makes a comparison to a factor or substance that is accepted as an “agonist” or “partial agonist” of the particular binding partner by those of skill in the art. Activation can be defined with respect to an increase in a particular effect or function that is induced by interaction of the agonist or partial agonist with a binding partner and can include allosteric effects. An “antagonist” can be a substance that inhibits its binding partner, typically a receptor. Inhibition is defined in the context of the particular assay, or can be apparent in the literature from a discussion herein that makes a comparison to a factor or substance that is accepted as an “antagonist” of the particular binding partner by those of skill in the art. Inhibition can be defined with respect to a decrease in a particular effect or function that is induced by interaction of the agonist with a binding partner, and can include allosteric effects.

As understood by those of skill in the art, assay methods for identifying compounds that modulate nicotinic AChR activity (e.g., agonists, antagonists, and partial agonists) generally require comparison to a control. One type of a “control” cell or “control” culture is a cell or culture that is treated substantially the same as the cell or culture exposed to the test compound, except the control culture is not exposed to test compound. For example, in methods that use voltage clamp electrophysiological procedures, the same cell can be tested in the presence and absence of test compound, by merely changing the external solution bathing the cell. Another type of “control” cell or “control” culture can be a cell or a culture of cells which are identical to the transfected cells, except the cells employed for the control culture do not express functional nicotinic AChRs. In this situation, the response of test cell to test compound is compared to the response (or lack of response) of receptor-negative (control) cell to test compound, when cells or cultures of each type of cell are exposed to substantially the same reaction conditions in the presence of compound being assayed.

As used herein, a human “α subunit” gene is a gene that encodes an α subunit of a human neuronal nicotinic acetylcholine receptor. The α subunit is a subunit of the nAChR to which ACh binds. Assignment of the name “α” to a putative nAChR subunit, according to Deneris et al., TIPS 12:34-40 (1991), is based on the conservation of adjacent cysteine residues in the presumed extracellular domain of the subunit that are the homologues of cysteines 192 and 193 of the Torpedo α subunit (see Noda et al. Nature 299:793-797(1982)). An α subunit also binds to ACh under physiological conditions and at physiological concentrations and, in the optional presence of a β subunit (i.e., some α subunits are functional alone, while others require the presence of a β subunit), generally forms a functional AChR as assessed by methods described herein or known to those of skill in this art.

Also contemplated are α subunits encoded by DNAs that encode α subunits as defined above, but that by virtue of degeneracy of the genetic code do not necessarily hybridize to the disclosed DNA under specified hybridization conditions. Such subunits also form a functional receptor, as assessed by the methods described herein or known to those of skill in the art, generally with one or more β subunit subtypes.

As used herein, a human “β subunit” gene is a gene that encodes a β subunit of a human neuronal nicotinic acetylcholine receptor. Assignment of the name “β” to a putative nAChR subunit, according to Deneris et al. supra, is based on the lack of adjacent cysteine residues (which are characteristic of α subunits). The β subunit is frequently referred to as the structural nAChR subunit (although it is possible that β subunits also have ACh binding properties). Combination of β subunit(s) with appropriate α subunit(s) leads to the formation of a functional receptor (for those α subunits that require a β subunit).

Also contemplated are β subunits encoded by DNAs that encode β subunits as defined above, but that by virtue of degeneracy of the genetic code do not necessarily hybridize to the disclosed DNA or deposited clones under the specified hybridization conditions. Such subunits also form functional receptors, as assessed by the methods described herein or known to those of skill in the art, in combination with appropriate α subunit subtype(s).

Nucleic Acid Molecules

As is known in the art for any DNA sequence determined by an automated approach, any nucleotide sequence disclosed herein may contain some errors. Nucleotide sequences determined by automation are typically at least about 90% identical, more typically at least about 95% to at least about 99.9% identical to the actual nucleotide sequence of the sequenced DNA molecule. The actual sequence can be more precisely determined by other approaches including manual DNA sequencing methods well known in the art. As is also known in the art, a single insertion or deletion in a determined nucleotide sequence compared to the actual sequence will cause a frame shift in translation of the nucleotide sequence such that the predicted amino acid sequence encoded by a determined nucleotide sequence will be completely different from the amino acid sequence actually encoded by the sequenced DNA molecule, beginning at the point of such an insertion or deletion.

Using the information provided herein, such as the nucleotide sequences in FIGS. 2, 4, and 6 (SEQ ID NOS: 6, 8, and 10, respectively), a nucleic acid molecule of the present invention encoding an chimeric nAChR subunit polypeptide can be obtained using standard cloning and screening procedures, such as those for cloning cDNAs using mRNA as starting material.

As indicated, nucleic acid molecules of the present invention can be in the form of RNA, such as mRNA, or in the form of DNA, including, for instance, cDNA and genomic DNA obtained by cloning or produced synthetically. The DNA can be double-stranded or single-stranded. Single-stranded DNA or RNA can be the coding strand, also known as the sense strand, or it can be the non-coding strand, also referred to as the anti-sense strand.

By “isolated” nucleic acid molecule(s) is intended a nucleic acid molecule, DNA or RNA, which has been removed from its native environment. For example, recombinant DNA molecules contained in a vector are considered isolated for the purposes of the present invention. Further examples of isolated DNA molecules include recombinant DNA molecules maintained in heterologous host cells or purified (partially or substantially) DNA molecules in solution. Isolated RNA molecules include in vivo or in vitro RNA transcripts of the DNA molecules of the present invention. Isolated nucleic acid molecules according to the present invention further include such molecules produced synthetically.

Isolated nucleic acid molecules of the present invention include DNA molecules comprising an open reading frame (ORF) shown in FIGS. 2, 4, and 6 (SEQ ID NOS: 6, 8, and 10, respectively); DNA molecules comprising the coding sequence for the chimeric nAChR subunit protein shown in FIGS. 3, 5, and 7 (SEQ ID NOS: 7, 9, and 11, respectively); and DNA molecules which comprise a sequence substantially different from those described above but which, due to the degeneracy of the genetic code, still encode the chimeric nAChR subunit protein. Of course, the genetic code is well known in the art. Thus, it would be routine for one skilled in the art to generate such degenerate variants. The present invention also includes other nucleic acid molecules and polypeptides defined according to the structural and functional requirements as disclosed herein.

Nucleic acids encoding portions of the chimeric nAChR subunit include nucleic acids determined by hybridization to those nucleic acids disclosed herein. Accordingly, the invention provides an isolated nucleic acid molecule comprising a polynucleotide which hybridizes under stringent hybridization conditions to a portion of the polynucleotide in a nucleic acid molecule of the invention described above, for instance, the polynucleotides disclosed in FIGS. 2, 4, and 6 (SEQ ID NOS: 6, 8, and 10, respectively). By “stringent hybridization conditions” is intended overnight incubation at 42° C. in a solution comprising: 50% formamide, 5× SSC (750 mM NaCl, 75 mM trisodium citrate), 50 mM sodium phosphate (pH 7.6), 5× Denhardt's solution, 10% dextran sulfate, and 20 μg/ml denatured, sheared salmon sperm DNA, followed by washing the filters in 0.1× SSC at about 65° C.

As indicated, nucleic acid molecules of the present invention that encode an chimeric nAChR subunit polypeptide can include, but are not limited to, those encoding the amino acid sequence of the polypeptide, by itself; the coding sequence for the polypeptide and additional sequences, such as those encoding a leader or secretory sequence, such as a pre-, or pro- or prepro-protein sequence; the coding sequence of the polypeptide, with or without the aforementioned additional coding sequences, together with additional, non-coding sequences, including for example, but not limited to introns and non-coding 5′ and 3′ sequences, such as the transcribed, non-translated sequences that play a role in transcription, mRNA processing, including splicing and polyadenylation signals, for example-ribosome binding and stability of mRNA; an additional coding sequence which codes for additional amino acids, such as those which provide additional functionalities. Thus, the sequence encoding the polypeptide can be fused to a marker sequence, such as a sequence encoding a peptide which facilitates purification of the fused polypeptide. In certain preferred embodiments of this aspect of the invention, the marker amino acid sequence is a hexa-histidine peptide, such as the tag provided in a pQE vector (Qiagen, Inc.), among others, many of which are commercially available. As described in Gentz et al., Proc. Natl. Acad. Sci. USA 86:821-824 (1989), for instance, hexa-histidine provides for convenient purification of the fusion protein. The “HA” tag is another peptide useful for purification which corresponds to an epitope derived from the influenza hemagglutinin protein, which has been described by Wilson et al., Cell 37: 767 (1984). As discussed below, other such fusion proteins include the chimeric nAChR subunit fused to Fc at the N- or C-terminus. The FLAG peptide (AspTyrLysAspAspAspAspLys) (Sigma Chemical Company, St Louis, Mo.), commonly used for the isolation, purification, and detection of recombinant proteins expressed in E. coli, has been used in particular examples of the present invention, e.g. the chimeric α4 receptor as disclosed herein.

The present invention further relates to variants of the nucleic acid molecules of the present invention, which encode portions, analogs or derivatives of the chimeric nAChR subunit protein. Variants can occur naturally, such as a natural allelic variant. By an “allelic variant” is intended one of several alternate forms of a gene occupying a given locus on a chromosome of an organism. Genes II, Lewin, B., ed., John Wiley & Sons, New York (1985). Non-naturally occurring variants can be produced using art-known mutagenesis techniques.

Such variants include those produced by nucleotide substitutions, deletions or additions which can involve one or more nucleotides. The variants can be altered in coding regions, non-coding regions, or both. Alterations in the coding regions can produce conservative or non-conservative amino acid substitutions, deletions or additions. Especially preferred among these are silent substitutions, additions and deletions, which do not alter the properties and activities of the chimeric nAChR subunit protein or portions thereof. Also especially preferred in this regard are conservative substitutions.

Further embodiments of the invention include isolated nucleic acid molecules comprising a polynucleotide having a nucleotide sequence at least 90% identical, and more preferably at least 95%, 96%, 97%, 98% or 99% identical to (a) a nucleotide sequence encoding the chimeric nAChR subunit polypeptide having the complete amino acid sequence in FIG. 3, 5, or 7 (SEQ ID NOS: 7, 9, or 11, respectively) or any other sequence according to the present invention; (b) a nucleotide sequence encoding the chimeric nAChR subunit polypeptide having the amino acid sequence in FIGS. 1A and B (SEQ ID NO:2), but lacking the N-terminal methionine; (c) a nucleotide sequence encoding a portion of the chimeric nAChR subunit polypeptide from a source sequence in accordance with the disclosed structure of the chimeric subunit, i.e. a nucleotide sequence meeting the above criteria relative to the native nAChR subunit component or the substituted portion based on comparison to the sequence of the native source sequence; and (h) a nucleotide sequence complementary to any of the nucleotide sequences in (a), (b), (c), (d), (e), (f) or (g) above.

By a polynucleotide having a nucleotide sequence at least, for example, 95% “identical” to a reference nucleotide sequence encoding an chimeric nAChR subunit polypeptide is intended that the nucleotide sequence of the polynucleotide is identical to the reference sequence except that the polynucleotide sequence can include up to five point mutations per each 100 nucleotides of the reference nucleotide sequence encoding the chimeric nAChR subunit polypeptide. In other words, to obtain a polynucleotide having a nucleotide sequence at least 95% identical to a reference nucleotide sequence, up to 5% of the nucleotides in the reference sequence can be deleted or substituted with another nucleotide, or a number of nucleotides up to 5% of the total nucleotides in the reference sequence can be inserted into the reference sequence. These mutations of the reference sequence can occur at the 5′ or 3′ terminal positions of the reference nucleotide sequence or anywhere between those terminal positions, interspersed either individually among nucleotides in the reference sequence or in one or more contiguous groups within the reference sequence.

As a practical matter, whether any particular nucleic acid molecule is at least 90%, 95%, 96%, 97%, 98% or 99% identical to, for instance, the nucleotide sequence of SEQ ID NOS: 6, 8, or 10, can be determined conventionally using known computer programs such as the Bestfit program (Wisconsin Sequence Analysis Package, Version 8 for Unix, Genetics Computer Group, University Research Park, 575 Science Drive, Madison, Wis. 53711). Bestfit uses the local homology algorithm of Smith and Waterman, Advances in Applied Mathematics 2: 482-489 (1981), to find the best segment of homology between two sequences. When using Bestfit or any other sequence alignment program to determine whether a particular sequence is, for instance, 95% identical to a reference sequence according to the present invention, the parameters are set, of course, such that the percentage of identity is calculated over the full length of the reference nucleotide sequence and that gaps in homology of up to 5% of the total number of nucleotides in the reference sequence are allowed.

By a polynucleotide having a nucleotide sequence at least, for example, 95% “identical” to a reference nucleotide sequence of the present invention, it is intended that the nucleotide sequence of the polynucleotide is identical to the reference sequence except that the polynucleotide sequence can include up to five point mutations per each 100 nucleotides of the reference nucleotide sequence encoding the chimeric nAChR subunit polypeptide. In other words, to obtain a polynucleotide having a nucleotide sequence at least 95% identical to a reference nucleotide sequence, up to 5% of the nucleotides in the reference sequence can be deleted or substituted with another nucleotide, or a number of nucleotides up to 5% of the total nucleotides in the reference sequence can be inserted into the reference sequence. The query sequence can be an entire sequence shown in FIG. 2, 4, or 6 (SEQ ID NOS: 6, 8, or 10, respectively), the ORF (open reading frame), or any fragment specified as described herein, e.g. domains of the native nAChR or the portion substituted to form the chimera.

As a practical matter, whether any particular nucleic acid molecule or polynucleotide is at least 90%, 95%, 96%, 97%, 98% or 99% identical to a nucleotide sequence of the presence invention can be determined conventionally using known computer programs. A preferred method for determining the best overall match between a query sequence (a sequence of the present invention) and a subject sequence, also referred to as a global sequence alignment, can be determined using the FASTDB computer program based on the algorithm of Brutlag et al. (Comp. App. Biosci. 6:237-245 (1990)). In a sequence alignment the query and subject sequences are both DNA sequences. An RNA sequence can be compared by converting U's to T's. The result of said global sequence alignment is in percent identity. Preferred parameters used in a FASTDB alignment of DNA sequences to calculate percent identity are: Matrix=Unitary, k-tuple=4, Mismatch Penalty=1, Joining Penalty=30, Randomization Group Length=0, Cutoff Score=1, Gap Penalty=5, Gap Size Penalty 0.05, Window Size=500 or the length of the subject nucleotide sequence, whichever is shorter.

If the subject sequence is shorter than the query sequence because of 5′ or 3′ deletions, not because of internal deletions, a manual correction must be made to the results. This is because the FASTDB program does not account for 5′ and 3′ truncations of the subject sequence when calculating percent identity. For subject sequences truncated at the 5′ or 3′ ends, relative to the query sequence, the percent identity is corrected by calculating the number of bases of the query sequence that are 5′ and 3′ of the subject sequence, which are not matched/aligned, as a percent of the total bases of the query sequence. Whether a nucleotide is matched/aligned is determined by results of the FASTDB sequence alignment. This percentage is then subtracted from the percent identity, calculated by the above FASTDB program using the specified parameters, to arrive at a final percent identity score. This corrected score is what is used for the purposes of the present invention. Only bases outside the 5′ and 3′ bases of the subject sequence, as displayed by the FASTDB alignment, which are not matched/aligned with the query sequence, are calculated for the purposes of manually adjusting the percent identity score.

For example, a 90 base subject sequence is aligned to a 100 base query sequence to determine percent identity. The deletions occur at the 5′ end of the subject sequence and therefore, the FASTDB alignment does not show a match/alignment of the first 10 bases at the 5′ end. The 10 unpaired bases represent 10% of the sequence (number of bases at the 5′ and 3′ ends not matched/total number of bases in the query sequence) so 10% is subtracted from the percent identity score calculated by the FASTDB program. If the remaining 90 bases were perfectly matched the final percent identity would be 90%. In another example, a 90 base subject sequence is compared with a 100 base query sequence. This time the deletions are internal deletions so that there are no bases on the 5′ or 3′ of the subject sequence which are not matched/aligned with the query. In this case the percent identity calculated by FASTDB is not manually corrected. Once again, only bases 5′ and 3′ of the subject sequence which are not matched/aligned with the query sequence are manually corrected for. No other manual corrections are to made for the purposes of the present invention.

Of course, due to the degeneracy of the genetic code, one of ordinary skill in the art will immediately recognize that a large number of the nucleic acid molecules having a sequence at least 90%, 95%, 96%, 97%, 98%, or 99% identical to the nucleic acid sequence of the nucleic acid sequence shown in FIGS. 2, 4, and 6 (SEQ ID NOS: 6, 8, or 10, respectively) will encode a polypeptide “having chimeric nAChR subunit protein activity.” In fact, because degenerate variants of these nucleotide sequences all encode the same polypeptide, this will be clear to the skilled artisan even without performing the above described comparison assay. It will be further recognized in the art that, for such nucleic acid molecules that are not degenerate variants, a reasonable number will also encode a polypeptide having chimeric nAChR subunit protein activity. This is because the skilled artisan is fully aware of amino acid substitutions that are either less likely or not likely to significantly effect protein function (e.g., replacing one aliphatic amino acid with a second aliphatic amino acid).

For example, guidance concerning how to make phenotypically silent amino acid substitutions is provided in Bowie, J. U. et al., “Deciphering the Message in Protein Sequences: Tolerance to Amino Acid Substitutions,” Science 247:1306-1310 (1990), wherein the authors indicate that proteins are surprisingly tolerant of amino acid substitutions.

Chimeric nAChR Subunit Polypeptides

It will be recognized in the art that some amino acid sequences of the chimeric nAChR subunit polypeptide can be varied without significant effect of the structure or function of the protein. If such differences in sequence are contemplated, it should be remembered that there will be critical areas on the protein which determine activity.

The examples of the polypeptides of the present invention, for example those shown in FIGS. 3, 5, and 7 (SEQ ID NOS: 7, 9, or 11, respectively), comprise an insertion derived from mouse serotonin type 3 receptors (m5HT3R). The illustrative insert is taken from the region of the mouse receptor that is considered somewhat structurally analogous to the corresponding region of the native nAChR receptor subunit that is being replaced.

In addition to the m5HT3 cytoplasmic +/−M3/M4 sequences used to produce a chimeric receptor according to the examples of the invention, e.g. as shown in the Figures, corresponding sequences can be switched between nicotinic receptor subunits. For example, internal sequences from subunits that assemble as homopentamers can be used as substituted sequences, e.g. internal sequences from the α7 subunit. Like the 5HT3 subunits, α7 subunits assemble well as homopentamers, indicating that their external and internal sequences are all compatible with such assembly. Further, the mouse 5HT3 subunit sequence from the “B” variant can also be used (the sequence disclosed in the figures herein is from the “A” variant) (Hanna, M. C., et al., Evidence for expression of heteromeric serotonin 5-HT(3) receptors in rodents. J. Neurochem. 75 (1):240-247 (2000)). In fact, any cytoplasmic +/−M3/M4 sequences from any member of the 4-transmembrane domain family of subunits including from GABA-A or glycine receptors as well as 5HT3 and nAChR subunits can be used (Buckle, V. J. et al., Chromosomal localization of GABAA receptor subunit genes: relationship to human genetic disease. Neuron 3 (5):647-654 (1989); Pierce, K. D., et al., A nonsense mutation in the α1 subunit of the inhibitory glycine receptor associated with bovine myoclonus. Mol. Cell. Neurosci. 17 (2):354-363 (2001)).

It will further be appreciated that, depending on the criteria used, concerning the exact “address” of the extracellular, intracellular and transmembrane domains of the chimeric nAChR subunit polypeptide differ slightly. For example, the exact location of the chimeric nAChR subunit extracellular domains illustrated schematically in FIG. 9 can vary slightly (e.g., the address can “shift” by about 1 to 5 residues) depending on the criteria used to define the domain.

Thus, the invention further includes variations of the chimeric nAChR subunit polypeptide which show substantial chimeric nAChR subunit polypeptide activity or which include regions of chimeric nAChR subunit protein such as the protein portions discussed below. Such mutants include deletions, insertions, inversions, repeats, and type substitutions. As indicated above, guidance concerning which amino acid changes are likely to be phenotypically silent can be found in Bowie, J. U., et al., “Deciphering the Message in Protein Sequences: Tolerance to Amino Acid Substitutions,” Science 247:1306-1310 (1990).

Thus, the fragment, derivative or analog of the polypeptide of FIG. 3, 5, or 7 (SEQ ID NOS: 7, 9, or 11, respectively) or any other sequence according to the present invention, can be (i) one in which one or more of the amino acid residues (e.g., 3, 5, 8, 10, 15 or 20) are substituted with a conserved or non-conserved amino acid residue (preferably a conserved amino acid residue) and such substituted amino acid residue may or may not be one encoded by the genetic code, or (ii) one in which one or more of the amino acid residues includes a substituent group (e.g., 3, 5, 8, 10, 15 or 20), or (iii) one in which the mature polypeptide is fused with another compound, such as a compound to increase the half-life of the polypeptide (for example, polyethylene glycol), or (iv) one in which the additional amino acids are fused to the mature polypeptide, such as an IgG Fc fusion region peptide or leader or secretory sequence or a sequence which is employed for purification of the mature polypeptide or a proprotein sequence. Such fragments, derivatives and analogs are deemed to be within the scope of those skilled in the art from the teachings herein.

Of particular interest are substitutions of charged amino acids with another charged amino acid and with neutral or negatively charged amino acids. The latter results in proteins with reduced positive charge to improve the characteristics of the chimeric nAChR subunit protein. The prevention of aggregation is highly desirable. Aggregation of proteins not only results in a loss of activity but can also be problematic when preparing pharmaceutical formulations, because they can be immunogenic. (Pinckard et al., Clin Exp. Immunol. 2:331-340 (1967); Robbins et al., Diabetes 36:838-845 (1987); Cleland et al. Crit. Rev. Therapeutic Drug Carrier Systems 10:307-377 (1993)).

The replacement of amino acids can also change the selectivity of binding to cell surface receptors. Ostade et al., Nature 361:266-268 (1993) describes certain mutations resulting in selective binding of TNF-α to only one of the two known types of TNF receptors. Thus, the chimeric nAChR subunit receptor of the present invention can include one or more (e.g., 3, 5, 8, 10, 15 or 20) amino acid substitutions, deletions or additions, either from natural mutations or human manipulation.

As indicated, changes are preferably of a minor nature, such as conservative amino acid substitutions that do not significantly affect the folding or activity of the protein (see below):

Conservative Amino Acid Substitutions

Aromatic:	Phenylalanine, Tryptophan, Tyrosine
Hydrophobic:	Leucine, Isoleucine, Valine
Polar:	Glutamine, Asparagine
Polar Hydroxyl:	Serine, Threonine
Basic:	Arginine, Lysine, Histidine
Acidic:	Aspartic Acid, Glutamic Acid
Small:	Alanine, Serine, Threonine, Methionine, Glycine

Of course, the number of amino acid substitutions a skilled artisan would make depends on many factors, including those described above. Generally speaking, the number of substitutions for any given chimeric subunit polypeptide will not be more than 50, 40, 30, 25, 20, 15, 10, 5 or 3.

Amino acids in the chimeric nAChR subunit protein of the present invention that are essential for function can be identified by methods known in the art, such as site-directed mutagenesis or alanine-scanning mutagenesis (Cunningham and Wells, Science 244:1081-1085 (1989)). The latter procedure introduces single alanine mutations at every residue in the molecule. The resulting mutant molecules are then tested for biological activity such as receptor binding or in vitro, or in vitro proliferative activity. Sites that are critical for ligand-receptor binding can also be determined by structural analysis such as crystallization, nuclear magnetic resonance or photoaffinity labeling (Smith et al., J. Mol. Biol. 224:399-904 (1992) and de Vos et al. Science 255:306-312 (1992)).

Even if deletion of one or more amino acids from the N-terminus of a protein results in modification or loss of one or more biological functions of the protein, other biological activities can still be retained. Thus, the ability of shortened chimeric nAChR subunit muteins to induce and/or bind to antibodies which recognize the complete or mature forms of the polypeptides generally will be retained when less than the majority of the residues of the complete or mature polypeptide are removed from the N-terminus. Whether a particular polypeptide lacking N-terminal residues of a complete polypeptide retains such immunologic activities can readily be determined by routine methods described herein and otherwise known in the art. It is not unlikely that a chimeric nAChR subunit mutein with a large number of deleted N-terminal amino acid residues can retain some biological or immunogenic activities. In fact, peptides composed of as few as six chimeric nAChR subunit amino acid residues can often evoke an immune response.

As mentioned above, even if deletion of one or more amino acids from the C-terminus of a protein results in modification or loss of one or more biological functions of the protein, other biological activities can still be retained. Thus, the ability of the shortened chimeric nAChR subunit mutein to induce and/or bind to antibodies which recognize the complete or mature forms of the polypeptide generally will be retained when less than the majority of the residues of the complete or mature polypeptide are removed from the C-terminus. Whether a particular polypeptide lacking C-terminal residues of a complete polypeptide retains such immunologic activities can readily be determined by routine methods described herein and otherwise known in the art. It is not unlikely that a chimeric nAChR subunit mutein with a large number of deleted C-terminal amino acid residues can retain some biological or immunogenic activities. In fact, peptides composed of as few as six chimeric nAChR subunit amino acid residues can often evoke an immune response.

Accordingly, the present invention further provides polypeptides having one or more residues deleted from the carboxy terminus of the amino acid sequence of the chimeric nAChR subunit polypeptide shown in FIG. 3, 5, or 7 (SEQ ID NOS: 7, 9, or 11, respectively) or any other sequence according to the present invention, and polynucleotides encoding such polypeptides. The invention also provides polypeptides having one or more amino acids deleted from both the amino and the carboxyl termini of a chimeric nAChR subunit polypeptide. It is also contemplated that polypeptides useful in production of the “isolated polypeptides” of the invention can produced by solid phase synthetic methods. See Houghten, R. A., Proc. Natl. Acad. Sci. USA 82:5131-5135 (1985); and U.S. Pat. No. 4,631,211 to Houghten et al. (1986).

The polypeptides of the present invention are preferably provided in an isolated form. By “isolated polypeptide” is intended a polypeptide removed from its native environment. Thus, a polypeptide produced and/or contained within a recombinant host cell is considered isolated for purposes of the present invention. Also intended as an “isolated polypeptide” are polypeptides that have been purified, partially or substantially, from a recombinant host. For example, a recombinantly produced version of the chimeric nAChR subunit polypeptide can be substantially purified by the one-step method described in Smith and Johnson, Gene 67:31-40 (1988).

By a polypeptide having an amino acid sequence at least, for example, 95% “identical” to a reference amino acid sequence of an chimeric nAChR subunit polypeptide is intended that the amino acid sequence of the polypeptide is identical to the reference sequence except that the polypeptide sequence can include up to five amino acid alterations per each 100 amino acids of the reference amino acid of the chimeric nAChR subunit polypeptide. In other words, to obtain a polypeptide having an amino acid sequence at least 95% identical to a reference amino acid sequence, up to 5% of the amino acid residues in the reference sequence can be deleted or substituted with another amino acid, or a number of amino acids up to 5% of the total amino acid residues in the reference sequence can be inserted into the reference sequence. These alterations of the reference sequence can occur at the amino or carboxy terminal positions of the reference amino acid sequence or anywhere between those terminal positions, interspersed either individually among residues in the reference sequence or in one or more contiguous groups within the reference sequence.

As a practical matter, whether any particular polypeptide is at least 90%, 95%, 96%, 97%, 98% or 99% identical to, for instance, the amino acid sequence shown in FIG. 3, 5, or 7 (SEQ ID NOS: 7, 9, or 11, respectively) or any other sequence according to the present invention, can be determined conventionally using known computer programs such the Bestfit program (Wisconsin Sequence Analysis Package, Version 8 for Unix, Genetics Computer Group, University Research Park, 575 Science Drive, Madison, Wis. 53711). When using Bestfit or any other sequence alignment program to determine whether a particular sequence is, for instance, 95% identical to a reference sequence according to the present invention, the parameters are set, of course, such that the percentage of identity is calculated over the full length of the reference amino acid sequence and that gaps in homology of up to 5% of the total number of amino acid residues in the reference sequence are allowed.

By a polypeptide having an amino acid sequence at least, for example, 95% “identical” to a query amino acid sequence of the present invention, it is intended that the amino acid sequence of the subject polypeptide is identical to the query sequence except that the subject polypeptide sequence can include up to five amino acid alterations per each 100 amino acids of the query amino acid sequence. In other words, to obtain a polypeptide having an amino acid sequence at least 95% identical to a query amino acid sequence, up to 5% of the amino acid residues in the subject sequence can be inserted, deleted, (indels) or substituted with another amino acid. These alterations of the reference sequence can occur at the amino or carboxy terminal positions of the reference amino acid sequence or anywhere between those terminal positions, interspersed either individually among residues in the reference sequence or in one or more contiguous groups within the reference sequence.

As a practical matter, whether any particular polypeptide is at least 90%, 95%, 96%, 97%, 98% or 99% identical to, for instance, the amino acid sequences shown in FIG. 3, 5, or 7 (SEQ ID NOS: 7, 9, or 11, respectively) or any other sequence according to the present invention, can be determined conventionally using known computer programs. A preferred method for determining the best overall match between a query sequence (a sequence of the present invention) and a subject sequence, also referred to as a global sequence alignment, can be determined using the FASTDB computer program based on the algorithm of Brutlag et al. Comp. App. Biosci. 6:237-245 (1990). In a sequence alignment the query and subject sequences are either both nucleotide sequences or both amino acid sequences. The result of said global sequence alignment is in percent identity. Preferred parameters used in a FASTDB amino acid alignment are: Matrix=PAM 0, k-tuple=2, Mismatch Penalty-1, Joining Penalty=20, Randomization Group Length=0, Cutoff Score=1, Window Size=sequence length, Gap Penalty=5, Gap Size Penalty=0.05, Window Size=500 or the length of the subject amino acid sequence, whichever is shorter.

If the subject sequence is shorter than the query sequence due to N- or C-terminal deletions, not because of internal deletions, a manual correction must be made to the results. This is because the FASTDB program does not account for N- and C-terminal truncations of the subject sequence when calculating global percent identity. For subject sequences truncated at the N- and C-termini, relative to the query sequence, the percent identity is corrected by calculating the number of residues of the query sequence that are N- and C-terminal of the subject sequence, which are not matched/aligned with a corresponding subject residue, as a percent of the total bases of the query sequence. Whether a residue is matched/aligned is determined by results of the FASTDB sequence alignment. This percentage is then subtracted from the percent identity, calculated by the above FASTDB program using the specified parameters, to arrive at a final percent identity score. This final percent identity score is what is used for the purposes of the present invention. Only residues of the query (reference) sequence that extend past the N- or C-termini of the subject sequence are considered for the purposes of manually adjusting the percent identity score. That is, only residues which are not matched/aligned with the N- or C-termini of the query sequence are counted when manually adjusting the percent identity score.

For example, a 90 amino acid residue subject sequence is aligned with a 100 residue query sequence to determine percent identity. The deletion occurs at the N-terminus of the subject sequence and therefore, the FASTDB alignment does not show a match/alignment of the first 10 residues at the N-terminus. The 10 unpaired residues represent 10% of the sequence (number of residues at the N- and C-termini not matched/total number of residues in the query sequence) so 10% is subtracted from the percent identity score calculated by the FASTDB program. If the remaining 90 residues were perfectly matched the final percent identity would be 90%. In another example, a 90 residue subject sequence is compared with a 100 residue query sequence. This time the deletions are internal deletions so there are no residues at the N- or C-termini of the subject sequence which are not matched/aligned with the query. In this case the percent identity calculated by FASTDB is not manually corrected. Once again, only residue positions outside the N- and C-terminal ends of the subject sequence, as displayed in the FASTDB alignment, which are not matched/aligned with the query sequence are manually corrected for. No other manual corrections are to be made for the purposes of the present invention.

The invention encompasses chimeric nAChR subunit polypeptides which are differentially modified during or after translation, e.g., by glycosylation, acetylation, phosphorylation, amidation, derivatization by known protecting/blocking groups, proteolytic cleavage, linkage to an antibody molecule or other cellular ligand, etc. Any of numerous chemical modifications can be carried out by known techniques, including but not limited, to specific chemical cleavage by cyanogen bromide, trypsin, chymotrypsin, papain, V8 protease, NaBH₄; acetylation, formylation, oxidation, reduction; metabolic synthesis in the presence of tunicamycin; etc.

Additional post-translational modifications encompassed by the invention include, for example, e.g., N-linked or O-linked carbohydrate chains, processing of N-terminal or C-terminal ends), attachment of chemical moieties to the amino acid backbone, chemical modifications of N-linked or O-linked carbohydrate chains, and addition of an N-terminal methionine residue as a result of procaryotic host cell expression. The polypeptides can also be modified with a detectable label, such as an enzymatic, fluorescent, isotopic or affinity label to allow for detection and isolation of the protein.

Preparation of Chimeric Gene Fusions

Generally speaking, the chimera gene fusions can be engineered using several steps of Polymerase Chain Reaction (PCR). Stepwise details of genetic engineering for two types of chimera cDNA construction are illustrated and described in FIG. 11. In order to produce one kind of chimera having the general structure illustrated in FIG. 11(I), the experimental design illustrated in (I) is used to construct two discontinuous fragments, A and B. Then, the PCR operation is applied again but using different primer sets to fuse relevant protions of the A and B fragments into a continuous strand of cDNA having the ability to code for the desired chimeric subunit.

In order to produce another kind of chimera having the general structure illustrated in FIG. 11(II), the experimental design modified to construct three discontinuous fragments, A, B and C. Then, the PCR operation is applied again but using different primer sets to fuse relevant protions of the A and B fragments. Another round of the PCR operation is than conducted using another set of primers to fuse the A+B to the C fragment to produce a continuous strand of cDNA having the ability to code for the desired chimeric subunit.

The resulting chimeric cDNAs are then digested with restriction enzymes to confirm their structure and sequence and processed in a separate operation so that they can be cloned into an appropriate expression vector before being delivered into a host cell line, e.g. SH-EPI. In one embodiment, the fusion gene is inserted to the pcDNA3.1/hyg (Invitrogen Corporation, Carlsbad, Calif.) for α4 or pcDNA3.1/zeo (Invitrogen Corporation, Carlsbad, Calif.) for β2 and β4 cloning/mammalian expression vectors as Hind III-Xba I or EcoR I-XbaI fragment, respectively. The pcDNA expression vectors use the CMV promoter. Other useful expression systems are the pEF series having a hEF-1α promoter (such as pEF6/Myc-His) (Invitrogen Corporation, Carlsbad, Calif.). The pCEP4 episomal vector can also be used (Invitrogen Corporation, Carlsbad, Calif.). Additional information regarding vectors from Invitrogen Corporation is available at http://www.invitrogen.com. (also, see below regarding Vectors and Host Cells).

Vectors and Host Cells

The present invention also relates to vectors which include the isolated DNA molecules of the present invention, host cells which are genetically engineered with the recombinant vectors, and the production of chimeric nAChR subunit polypeptides or fragments thereof by recombinant techniques.

The polynucleotides can be joined to a vector containing a selectable marker for propagation in a host by using techniques that are known to those familiar in the art, and that generally involve restriction enzyme cleavage of the circular vector at a specific site in such a way that the polynucleotide of interest, sometimes also treated with restriction enzyme to optimally prepare it for the joining process, will anneal to the linearized vector via blunt-end or complementary end ligation. Typically, the unligated products are then separated from the ligated product of interest containing the polynucleotide of interest contained within the vector, and the isolated vector-polynucleotide complexes are cloned through a process involving transformation of bacteria prior to subsequent analysis to confirm presence of the polynucleotide and that it is in the correct orientation for subsequent expression. If the vector is a virus, it can be packaged in vitro using an appropriate packaging cell line so that the virus contains the polynucleotide of interest in the appropriate form and structure.

The DNA insert can be operatively linked to an appropriate promoter, such as the phage lambda PL promoter, the E. coli lac, trp and tac promoters, the SV40 early and late promoters and promoters of retroviral LTRs, to name a few. Other suitable promoters will be known to the skilled artisan. The expression constructs will further contain sites for transcription initiation, termination and, in the transcribed region, a ribosome binding site for translation. The coding portion of the mature transcripts expressed by the constructs will preferably include a translation initiating at the beginning and a termination codon (UAA, UGA or UAG) appropriately positioned at the end of the polypeptide to be translated.

As indicated, the expression vectors will preferably include at least one selectable marker. Such markers include dihydrofolate reductase or neomycin resistance for eukaryotic cell culture and tetracycline or ampicillin resistance genes for culturing in E. coli and other bacteria. Representative examples of appropriate hosts include, but are not limited to, bacterial cells, such as E. coli, Streptomyces and Salmonella typhimurium cells; fungal cells, such as yeast cells; insect cells such as Drosophila S2 and Spodoptera Sf9 cells; animal cells such as CHO, COS and Bowes melanoma cells; and plant cells. Appropriate culture mediums and conditions for the above-described host cells are known in the art.

Examples of mammalian expression vectors useful according to the present invention also include pTRE for regulated control of gene expression, e.g. pTRE HA (with HA epitope) BD Biosciences Clontech, Palo Alto, Calif. (for additional information see http://www.clontech.com/techinfo/vectors/vectorsT-Z/pTRE-HA.shtml). Cell line variants, e.g. large T-antigen-expressing HEK cells, allow high expression from the SV40 promoter in mammalian cells), and the 3′-poly-A signal can be tailored to optimize expression and translation efficiency.

Among known bacterial promoters suitable for use in the production of proteins of the present invention include the E. coli lacI and lacZ promoters, the T3 and T7 promoters, the gpt promoter, the lambda PR and PL promoters and the trp promoter. Suitable eukaryotic promoters include the CMV immediate early promoter, the HSV thymidine kinase promoter, the early and late SV40 promoters, the promoters of retroviral LTRs, such as those of the Rous Sarcoma Virus (RSV), and metallothionein promoters, such as the mouse metallothionein-I promoter.

The vectors used for prokaryotic expression also contain a Shine-Delgarno sequence 5′ to the AUG initiation codon. Shine-Delgarno sequences are short sequences generally located about 10 nucleotides up-stream (i.e., 5′) from the AUG initiation codon. These sequences essentially direct prokaryotic ribosomes to the AUG initiation codon.

Thus, the present invention is also directed to expression vector useful for the production of the proteins of the present invention. Among vectors preferred for use in bacteria include pQE70, pQE60 and pQE-9, available from Qiagen; pBS vectors, Phagescript vectors, Bluescript vectors, pNH8A, pNH16a, pNH18A, pNH46A, available from Stratagene; and ptrc99a, pKK223-3, pKK233-3, pDR540, pRIT5 available from Pharmacia. Among preferred eukaryotic vectors are pWLNEO, pSV2CAT, pOG44, pXT1 and pSG available from Stratagene; and pSVK3, pBPV, pMSG and pSVL available from Pharmacia. Other suitable vectors will be readily apparent to the skilled artisan.

Introduction of the construct into the host cell can be effected by calcium phosphate transfection, DEAE-dextran mediated transfection, cationic lipid-mediated transfection, electroporation, transduction, infection or other methods. Such methods are described in many standard laboratory manuals, such as Davis et al., Basic Methods In Molecular Biology (1986). If the vector is a virus, it can be packaged in vitro using an appropriate packaging cell line and then transduced into host cells.

The polypeptide can be expressed in a modified form, such as a fusion protein, and can include not only secretion signals, but also additional heterologous functional regions. For instance, a region of additional amino acids, particularly charged amino acids, can be added to the N-terminus of the polypeptide to improve stability and persistence in the host cell, during purification, or during subsequent handling and storage. Also, peptide moieties can be added to the polypeptide to facilitate purification. Such regions can be removed prior to final preparation of the polypeptide. The addition of peptide moieties to polypeptides to engender secretion or excretion, to improve stability and to facilitate purification, among others, are familiar and routine techniques in the art. A preferred fusion protein comprises a heterologous region from immunoglobulin that is useful to solubilize proteins. For example, EP-A-O 464 533 (Canadian counterpart 2045869) discloses fusion proteins comprising various portions of constant region of immunoglobin molecules together with another human protein or part thereof. In many cases, the Fc part in a fusion protein is thoroughly advantageous for use in therapy and diagnosis and thus results, for example, in improved pharmacokinetic properties (EP-A 0232 262). On the other hand, for some uses it would be desirable to be able to delete the Fc part after the fusion protein has been expressed, detected and purified in the advantageous manner described. This is the case when Fc portion proves to be a hindrance to use in therapy and diagnosis, for example when the fusion protein is to be used as antigen for immunizations. In drug discovery, for example, human proteins, such as, hIL5-receptor has been fused with Fc portions for the purpose of high-throughput screening assays to identify antagonists of hiL-5. See Bennett, D., et al., J. Mol. Recog., 8:52-58 (1995) and Johanson, K. et al., J. Biol. Chem., 270(16):9459-9471 (1995).

The chimeric nAChR subunit protein can be recovered and purified from recombinant cell cultures by well-known methods including ammonium sulfate or ethanol precipitation, acid extraction, anion or cation exchange chromatography, phosphocellulose chromatography, hydrophobic interaction chromatography, affinity chromatography, hydroxylapatite chromatography and lectin chromatography. Most preferably, high performance liquid chromatography (“HPLC”) is employed for purification. Polypeptides of the present invention include naturally purified products, products of chemical synthetic procedures, and products produced by recombinant techniques from a prokaryotic or eukaryotic host, including, for example, bacterial, yeast, higher plant, insect and mammalian cells. Depending upon the host employed in a recombinant production procedure, the polypeptides of the present invention can be glycosylated or can be non-glycosylated. In addition, polypeptides of the invention can also include an initial modified methionine residue, in some cases as a result of host-mediated processes.

In addition to encompassing host cells containing the vector constructs discussed herein, the invention also encompasses primary, secondary, and immortalized host cells of vertebrate origin, particularly mammalian origin, that have been engineered to delete or replace endogenous genetic material (e.g., chimeric nAChR subunit coding sequence), and/or to include genetic material (e.g., heterologous polynucleotide sequences) that is operably associated with chimeric nAChR subunit polynucleotides of the invention, and which activates, alters, and/or amplifies endogenous chimeric nAChR subunit polynucleotides. For example, techniques known in the art can be used to operably associate heterologous control regions (e.g., promoter and/or enhancer) and endogenous chimeric nAChR subunit polynucleotide sequences via homologous, recombination (see, e.g., U.S. Pat. No. 5,641,670, issued Jun. 24, 1997; International Publication No. WO 96/29411, published Sep. 26, 1996; International Publication No. WO 94/12650, published Aug. 4, 1994; Koller et al., Proc. Natl. Acad. Sci. USA 86:8932-8935 (1989); and Zijlstra et al., Nature 342:435-438 (1989), the disclosures of each of which are incorporated by reference in their entireties).

Appropriate cells include SH-EP type cells (e.g. SH-EP1), HEK-293 (human embryonic kidney), IMR-32 human neuroblastoma cells, and CATH.a mouse neuronal cells. (See Lukas, R. J., et al., “Some Methods of Studies of Nicotinic Acetylcholine Receptor Pharmacology,” Ch. 1, in Methods & New Frontiers in Neuroscience, CRC Press LLC (2002), and citations therein). SH-EP type cells can be obtained from the human neuroblastoma parental cell line SK-N-SH (See Ross, R. A., et al., “Coordinate Morphological and Biochemical Interconversion of Human Neuroblastoma Cells,” JNCI, 71(4):741-749 (1983)). SH-EP (“EP” for epithelial-like morphology) cells are morphologically distinguishable, lack expression of noradrenergic enzyme activity (tyrosine hydroxylase and dopamine-β-hydroxylase), and contain an isochromosome 1q (long arm of chromosome 1) (Ross, et al., 1983). Without wishing to be bound by any particular theory, it appears that such cells can be useful because they possess and properly express complex transmembrane proteins due to their neuroepithelial lineage, while having lost expression of native nAChRs which could otherwise complicate analysis involving heterologous expression of nAChRs. (See Lukas, R. J. et al., 2002).

Those of skill in the art will recognize that the suitability of particular cell lines for heterologous expression of nAChRs according to the invention can be determined empirically, and that the foregoing guidance will provide direction for development of useful models beyond the examples expressly provided herein.

Transgenic Non-Human Animals

The polypeptides of the invention can also be expressed in transgenic animals. Animals of any species, including, but not limited to, mice, rats, rabbits, hamsters, guinea pigs, pigs, micro-pigs, goats, sheep, cows and non-human primates, e.g., baboons, monkeys, and chimpanzees can be used to generate transgenic animals. In a specific embodiment, techniques described herein or otherwise known in the art, are used to express polypeptides of the invention in humans, as part of a gene therapy protocol.

Any technique known in the art can be used to introduce the transgene (i.e., polynucleotides of the invention) into animals to produce the founder lines of transgenic animals. Such techniques include, but are not limited to, pronuclear microinjection (Paterson et al., Appl. Microbiol. Biotechnol. 40:691-698 (1994); Carver et al., Biotechnology (NY) 11:1263-1270 (1993); Wright et al., Biotechnology (NY) 9:830-834(1991); and Hoppe et al., U.S. Pat. No. 4,873,191 (1989)); retrovirus mediated gene transfer into germ lines (Van der Putten et al., Proc. Natl. Acad. Sci., USA 82:6148-6152 (1985)), blastocysts or embryos; gene targeting in embryonic stem cells (Thompson et al., Cell 56:313-321 (1989)); electroporation of cells or embryos (Lo, Mol Cell. Biol. 3:1803-1814 (1983)); introduction of the polynucleotides of the invention using a gene gun (see, e.g., Ulmer et al., Science 259:1745 (1993); introducing nucleic acid constructs into embryonic pleuripotent stem cells and transferring the stem cells back into the blastocyst; and sperm-mediated gene transfer (Lavitrano et al., Cell 57:717-723 (1989); etc. For a review of such techniques, see Gordon, “Transgenic Animals,” Intl. Rev. Cytol. 115:171-229 (1989), which is incorporated by reference herein in its entirety. Further, the contents of each of the documents recited in this paragraph are herein incorporated by reference in their entirety.

Any technique known in the art can be used to produce transgenic clones containing polynucleotides of the invention, for example, nuclear transfer into enucleated oocytes of nuclei from cultured embryonic, fetal, or adult cells induced to quiescence (Campell et al., Nature 380:64-66 (1996); Wilmut et al., Nature 385:810-813 (1997)), each of which is herein incorporated by reference in its entirety).

The present invention provides for transgenic animals that carry the transgene in all their cells, as well as animals which carry the transgene in some, but not all their cells, i.e., mosaic animals or chimeric. The transgene can be integrated as a single transgene or as multiple copies such as in concatamers, e.g., head-to-head tandems or head-to-tail tandems. The transgene can also be selectively introduced into and activated in a particular cell type by following, for example, the teaching of Lasko et al., Proc. Natl. Acad. Sci. USA 89:6232-6236 (1992). The regulatory sequences required for such a cell-type specific activation will depend upon the particular cell type of interest, and will be apparent to those of skill in the art. When it is desired that the polynucleotide transgene be integrated into the chromosomal site of the endogenous gene, gene targeting is preferred. Briefly, when such a technique is to be utilized, vectors containing some nucleotide sequences homologous to the endogenous gene are designed for the purpose of integrating, via homologous recombination with chromosomal sequences, into and disrupting the function of the nucleotide sequence of the endogenous gene. The transgene can also be selectively introduced into a particular cell type, thus inactivating the endogenous gene in only that cell type, by following, for example, the teaching of Gu et al., Science 265:103-106 (1994). The regulatory sequences required for such a cell-type specific inactivation will depend upon the particular cell type of interest, and will be apparent to those of skill in the art. The contents of each of the documents recited in this paragraph are herein incorporated by reference in their entirety.

Once transgenic animals have been generated, the expression of the recombinant gene can be assayed utilizing standard techniques. Initial screening can be accomplished by Southern blot analysis or PCR techniques to analyze animal tissues to verify that integration of the transgene has taken place. The level of mRNA expression of the transgene in the tissues of the transgenic animals can also be assessed using techniques which include, but are not limited to, Northern blot analysis of tissue samples obtained from the animal, in situ hybridization analysis, and reverse transcriptase-PCR (rt-PCR). Samples of transgenic gene-expressing tissue can also be evaluated immunocytochemically or immunohistochemically using antibodies specific for the transgene product.

Once the founder animals are produced, they can be bred, inbred, outbred, or crossbred to produce colonies of the particular animal. Examples of such breeding strategies include, but are not limited to: outbreeding of founder animals with more than one integration site in order to establish separate lines; inbreeding of separate lines in order to produce compound transgenics that express the transgene at higher levels because of the effects of additive expression of each transgene; crossing of heterozygous transgenic animals to produce animals homozygous for a given integration site in order to both augment expression and eliminate the need for screening of animals by DNA analysis; crossing of separate homozygous lines to produce compound heterozygous or homozygous lines; and breeding to place the transgene on a distinct background that is appropriate for an experimental model of interest.

Having generally described the invention, the same will be more readily understood by reference to the following examples, which are provided by way of illustration only.

Xenopus oocytes are injected with in vitro transcripts prepared from constructs containing DNA encoding chimeric α4, β2, and β4 subunits. Electrophysiological measurements of the oocyte transmembrane currents are made using the two-electrode voltage clamp technique (see, e.g., Stuhmer, Meth. Enzymol. 207:319-339 (1992)).

1. Preparation of in vitro Transcripts

Recombinant capped transcripts of pCMV expression constructs are synthesized from linearized plasmids using the mCAP RNA Capping Kit (Cat. #200350 from Stratagene, Inc., La Jolla, Calif.). The mass of each synthesized transcript is determined by UV absorbance and the integrity of each transcript was determined by electrophoresis through an agarose gel.

2. Electrophysiology

Xenopus oocytes are injected with either 12.5, 50 or 125 ng of chimeric nAChR subunit transcript per oocyte. The preparation and injection of oocytes is carried out as described by Dascal in Crit. Rev. Biochem. 22:317-387 (1987). Two-to-six days following mRNA injection, the oocytes are examined using the two-electrode voltage clamp technique. The cells are bathed in Ringer's solution (115 mM NaCl, 2.5 mM KCl, 1.8 mM CaCl₂, 10 mM HEPES, pH 7.3) containing 1 μM atropine with or without 100 μM d-tubocurarine. Cells are voltage-clamped at −60 to −80 mV. Data are acquired with axotape software at 2-5 Hz. The agonists acetylcholine (ACh), nicotine, and cytisine are added at concentrations ranging from 0.1 μM to 100 μM.

The current response of injected oocytes to 10 μM ACh is also examined in terms of membrane voltage. In these experiments, voltage steps are applied to the cells in the presence of ACh. The contribution of Ca⁺⁺ flux to the total current can be ascertained by varying the calcium concentration in the external medium and taking multiple current measurements at different holding potentials around the reversal potential. Such studies indicate whether the channel carrying the current generated in response to ACh treatment of injected oocytes is permeable to Na⁺, K⁺ and Ca⁺⁺.

Human embryonic kidney (HEK) 293 cells are transiently and stably transfected with DNA encoding chimeric nAChR subunits. Transient transfectants were analyzed for expression of nicotinic AChR using various assays, e.g., electrophysiological methods, Ca⁺⁺-sensitive fluorescent indicator-based assays, and [¹²⁵I]-α-bungarotoxin-binding assays.

1. Transient Transfection of HEK Cells

About 2×10⁶ HEK cells are transiently transfected with 18 μg of the plasmid(s) bearing the chimeric subunit expression constructs according to standard CaPO₄ transfection procedures (Wigler et al., Proc. Natl. Acad. Sci. USA 76:1373-1376(1979)). In addition, 2 μg of plasmid pCMV βgal (Clontech Laboratories, Palo Alto, Calif.), which contains the Escherichia coli, β-galactosidase gene fused to the CMV promoter, are co-transfected as a reporter gene for monitoring the efficiency of transfection. The transfectants are analyzed for β-galactosidase expression by measurement of β-galactosidase activity (Miller, Experiments in Molecular Genetics, pp. 352-355, Cold Spring Harbor Press (1972)). Transfectants can also be analyzed for β-galactosidase expression by direct staining of the product of a reaction involving β-galactosidase and the X-gal substrate (Jones, EMBO 5:3133-3142(1986)).

2. Stable Transfection of HEK Cells

HEK cells are transfected using the calcium phosphate transfection procedure (Current Protocols in Molecular Biology, Vol. 1, Wiley Inter-Science, Supplement 14, Unit 9.1.1-9.1.9 (1990)). Ten-cm plates, each containing one-to-two million HEK cells are transfected with 1 ml of DNA/calcium phosphate precipitate containing 9.5 μg of each pCMV chimeric subunit expression construct and 1 μg pSV2neo (as a selectable marker). After 14 days of growth in media containing 1 μg/ml G418, colonies form and are individually isolated by using cloning cylinders. The isolates are subjected to limiting dilution and screened to identify those that expressed the highest level of nicotinic AChR, as described below.

3. Analysis of Transfectants

Activation of the ligand-gated nicotinic AChR by agonists leads to an influx of cations, including Ca⁺⁺, through the receptor channel. Ca⁺⁺ entry into the cell through the channel can induce release of calcium contained in intracellular stores. As one example, a receptor comprising a chimeric nAChR subunit of the invention (comprising chimeric subunits of α4 and β2) exhibited a two-fold increase in net current compared to the wild-type counterpart when exposed to a compound known to specifically activate α4β2 nAChR.

Monovalent cation entry into the cell through the channel can also result in an increase in cytoplasmic Ca⁺⁺ levels through depolarization of the membrane and subsequent activation of voltage-dependent calcium channels. Therefore, methods of detecting transient increases in intracellular calcium concentration can be applied to the analysis of functional nicotinic AChR expression. One method for measuring intracellular calcium levels relies on calcium-sensitive fluorescent indicators.

Calcium-sensitive indicators, such as fluo-3 (Catalog No. F-1241, Molecular Probes, Inc., Eugene, Oreg.), are available as acetoxymethyl esters which are membrane permeable. When the acetoxymethyl ester form of the indicator enters a cell, the ester group is removed by cytosolic esterases, thereby trapping the free indicator in the cytosol. Interaction of the free indicator with calcium results in increased fluorescence of the indicator; therefore, an increase in the intracellular Ca⁺⁺ concentration of cells containing the indicator can be expressed directly as an increase in fluorescence. An automated fluorescence detection system for assaying nicotinic AChR has been described in PCT Patent Application No. PCT/US92/11090.

HEK cells that have been transiently or stably co-transfected with DNA encoding chimeric subunits are analyzed for expression of functional recombinant nicotinic AChR using the automated fluorescent indicator-based assay. The assay procedure is as follows.

Untransfected HEK cells (or HEK cells transfected with expression vector alone) and HEK cells that have been co-transfected with expression constructs of the invention are plated in the wells of a 96-well microtiter dish and loaded with fluo-3 by incubation for 2 hours at 20° C. in a medium containing 20 μM fluo-3, 0.2% Pluronic F-127 in HBS (125 mM NaCl, 5 mM KCl, 1.8 mM CaCl₂, 0.62 mM MgSO₄, 6 mM glucose, 20 mM HEPES, pH 7.4). The cells are then washed with assay buffer (i.e., HBS). The antagonist d-tubocurarine is added to some of the wells at a final concentration of 10 μM. The microtiter dish is then placed into a fluorescence plate reader and the basal fluorescence of each well is measured and recorded before addition of 200 μM nicotine to the wells. The fluorescence of the wells is monitored repeatedly during a period of approximately 60 seconds following addition of nicotine.

The fluorescence of the untransfected HEK cells (or HEK cells transfected with vector along) should not change after addition of nicotine. In contrast, the fluorescence of the co-transfected cells, in the absence of d-tubocurarine, is expected to increase significantly after addition of nicotine to the wells. This nicotine-stimulated increase in fluorescence should not be observed in co-transfected cells exposed to the antagonist d-tubocurarine. These tests can demonstrate whether the co-transfected cells express functional recombinant AChR activated by nicotine and blocked by d-tubocurarine.

HEK293 cells transiently transfected with pCMV expression constructs according to the invention are analyzed for [¹²⁵I]-α-bungarotoxin (BgTx) binding. Whole transfected cells and membranes prepared from transfected cells are examined in these assays. Rat brain membranes are included in the assays as a positive control.

Rat brain membranes are prepared according to the method of Hampson et al., J. Neurochem 49:1209 (1987). Membranes were prepared from the HEK cells transfected with pCMV expression constructs and HEK cells transiently transfected with plasmid pUC19 only (negative control) according to the method of Perez-Reyes et al., Nature 340:233 (1989). Whole transfected and negative control cells are obtained by spraying the tissue culture plates with phosphate-buffered saline containing 0.1% (w/v) BSA. The cells are then centrifuged at low speed, washed once, resuspended in assay buffer (118 mM NaCl, 4.8 mM KCl, 2.5 mM CaCl₂, 1.2 mM MgSO₄, 20 mM HEPES, 0.1% (w/v) BSA, 0.05% (w/v) bacitracin and 0.5 mM PMSF, pH 7.5) and counted.

Specific binding of [¹²⁵I]-α-BgTx to rat brain membranes is determined essentially as described by Marks et al., Molec. Pharmacol. 22:554-564(1982), with several modifications. The membranes are washed twice in assay buffer. The assay is carried out in 12×75 mm polypropylene test tubes in a total volume of 0.5 ml assay buffer. The membranes are incubated with 10 nM [¹²⁵I]-α-BgTx (New England Nuclear, Boston, Mass.) for one hour at 37° C. The assay mixtures are then centrifuged at 2300×g for 10 minutes at 4° C. The supernatant is decanted and the pellets are washed twice with 2 ml aliquots of ice-cold assay buffer. The supernatants are decanted again and the radioactivity of the pellets is measured in a γ-counter. Nonspecific binding is determined in the presence of 1 μM unlabeled α-BgTx. Specific binding is determined by subtracting nonspecific binding from total binding. Specific binding of [¹²⁵I]-α-BgTx to membranes prepared from transfected and negative control cells is determined as described for determining specific binding to rat brain membranes except that the assay buffer does not contain BSA, bacitracin and PMSF. Specific binding of [¹²⁵I]-α-BgTx to transfected and negative control whole cells is determined basically as described for determining specific binding to rat brain membranes.

[¹²⁵I]-α-BgTx binding is evaluated as a function of membrane concentration and as a function of incubation time. [¹²⁵I]-α-BgTx binding to rat brain membranes is expected to increase in a linear fashion with increasing amounts of membrane (ranging between 25-500 μg).

To monitor [¹²⁵I]-α-BgTx binding to rat brain membranes and whole transfected and negative control cells, 300 μg of membrane or 500,000 cells are incubated with 1 nM or 10 nM [¹²⁵I]-α-BgTx, respectively, at 37° C. for various times ranging from 0-350 min. Aliquots of assay mixture are transferred to 1.5 ml microfuge tubes at various times and centrifuged. The pellets are washed twice with assay buffer.

Recombinant cell lines generated by transfection with DNA encoding chimeric nAChR subunits of the invention can be further characterized using one or more of the following methods.

A. Northern or Slot Blot Analysis for Expression of α- and/or β-Subunit Encoding Messages

Total RNA is isolated from about 1×10⁷ cells and 10-15 μg of RNA from each cell type is used for northern or slot blot hybridization analysis. The inserts from chimeric nAChR-encoding plasmids can be nick-translated and used as probe. In addition, the β-actin gene sequence (Cleveland et al., Cell 20:95-105 (1980)) can be nick-translated and used as a control probe on duplicate filters to confirm the presence or absence of RNA on each blot and to provide a rough standard for use in quantitating differences in α- or β-specific mRNA levels between cell lines. Typical northern and slot blot hybridization and wash conditions are as follows: hybridization in 5×SSPE, 5× Denhardt's solution, 50% formamide, at 42° C. followed by washing in 0.2×SSPE, 0.1% SDS, at 65° C.

B. Nicotine-Binding Assay

Cell lines generated by transfection with chimeric nAChR α- or α- and β-subunit-encoding DNA can be analyzed for their ability to bind nicotine, for example, as compared to control cell lines: neuronally-derived cell lines PC12 (Boulter et al., Nature 319(6052):368-74 (1986); ATCC #CRL1721) and IMR32 (Clementi, et al. Int. J. Neurochem. 47:291-297(1986); ATCC #CCL127), and muscle-derived cell line BC3H1 (Patrick, et al., J. Biol. Chem. 252:2143-2153 (1977)). Negative control cells (i.e., host cells from which the transfectants are prepared) are also included in the assay. The assay is conducted as follows:

Just prior to being assayed, transfected cells are removed from plates by scraping. Positive control cells used are PC12, BC3H1, and IMR32 (which have been starved for fresh media for seven days). Control cell lines are removed by rinsing in 37° C. assay buffer (50 mM Tris/HCl, 1 mM MgCl₂, 2 mM CaCl₂, 120 mM NaCl, 3 mM EDTA, 2 mg/ml BSA and 0.1% aprotinin at pH 7.4). The cells are washed and resuspended to a concentration of 1×10⁶/250 μl. To each plastic assay tube is added 250 μl of the cell solution, 15 nM ³H-nicotine, with or without 1 mM unlabeled nicotine, and assay buffer to make a final volume of 500 μl. The assays for the transfected cell lines are incubated for 30 min at room temperature; the assays of the positive control cells are incubated for 2 min at 1° C. After the appropriate incubation time, 450 μl aliquots of assay volume are filtered through Whatman GF/C glass fiber filters which has been pretreated by incubation in 0.05% polyethyleneimine for 24 hours at 4° C. The filters are then washed twice, with 4 ml each wash, with ice cold assay buffer. After washing, the filters are dried, added to vials containing 5 ml scintillation fluid and radioactivity is measured.

C. ⁸⁶Rb Ion-Flux Assay

The ability of nicotine or nicotine agonists and antagonists to mediate the influx of ⁸⁶Rb into transfected and control cells has been found to provide an indication of the presence of functional AChRs on the cell surface. The ⁸⁶Rb ion-flux assay is conducted as follows:

1. The night before the experiment, cells are plated at 2×10⁶ per well (i.e., 2 ml per well) in a 6-well polylysine-coated plate.

2. The culture medium is decanted and the plate washed with 2 ml of assay buffer (50 mM HEPES, 260 mM sucrose, 5.4 mM KCl, 1.8 mM CaCl₂, 0.8 mM MgSO₄, 5.5 mM glucose) at room temperature.

3. The assay buffer is decanted and 1 ml of assay buffer, containing 3 μCi/ml ⁸⁶Rb, with 5 mM ouabain and agonist or antagonist in a concentration to effect a maximum response is added.

4. The plate is incubated on ice at 1° C. for 4 min.

5. The buffer is decanted into a waste container and each well was washed with 3 ml of assay buffer, followed by two washes of 2 ml each.

6. The cells are lysed with 2×0.5 ml of 0.2% SDS per well and transferred to a scintillation vial containing 5 ml of scintillation fluid.

7. The radioactivity contained in each vial is measured and the data calculated.

D. Electrophysiological Analysis of Mammalian Cells Transfected with Chimeric nAChR Subunit-Encoding DNA

Electrophysiological measurements can be used to assess the activity of recombinant receptors or to assess the ability of a test compound to potentiate, antagonize or otherwise modulate the magnitude and duration of the flow of cations through the ligand-gated recombinant AChR. The function of the expressed neuronal AChR can be assessed by a variety of electrophysiological techniques, including two-electrode voltage clamp and patch clamp methods. The cation-conducting channel intrinsic to the AChR opens in response to acetylcholine (ACh) or other nicotinic cholinergic agonists, permitting the flow of transmembrane current carried predominantly by sodium and potassium ions under physiological conditions. This current can be monitored directly by voltage clamp techniques. In preferred embodiments, transfected mammalian cells or injected oocytes are analyzed electrophysiologically for the presence of AChR agonist-dependent currents.

Having hereby disclosed the subject matter of the present invention, it should be apparent that many modifications, substitutions, and variations of the present invention are possible in light thereof. It is to be understood that the present invention can be practiced other than as specifically described. Such modifications, substitutions and variations are intended to be within the scope of the present application.